Many prior art gravel packing methods and devices are incapable of completely packing a well penetrating an unconsolidated or poorly consolidated subterranean oil and gas reservoir. With such methods and devices, the annulus between the sand screen and the casing for in-casing gravel packs or between the screen and the side of the hole for open hole or under-reamed gravel packs are not completely packed. The problem of incomplete packing often occurs as a result of the formation of sand “bridges” in the interval to be packed, which prevents the placement of sufficient sand below the bridge, in the case of top down gravel packing, or above the bridge in the case of bottom up gravel packing.
One prior art method, which seeks to solve the problem of sand bridge formation, can be found in U.S. Pat. No. 4,945,991 issued to Lloyd G. Jones and assigned to Mobil Oil Corporation (“the '991 patent”). This method employs one or more shunt tubes, which are mounted to the exterior of the sand screen. The shunt tubes extend substantially throughout the distance of the annulus to be gravel packed and can be open to fluids at both ends or open at the top and sealed at the bottom. In one embodiment, the shunt tubes are provided with perforations at pre-selected intervals along their length to establish fluid communication between the shunt tubes and the annulus, so that if a sand or gravel bridge is formed, the space blocked by the bridge can be filled. In another embodiment of the invention, the slurry is ejected into the annulus through lateral conduits, which are attached to and spaced along the length of the shunt tubes.
The purported method according to the '991 patent includes the following steps. First, a borehole is formed through the reservoir and lined with a casing. Next, the casing is perforated at preselected intervals to form perforation tunnels adjacent a substantial portion of the reservoir. The sand screen is then placed inside of the casing next to the perforation tunnels. The annulus is formed between the sand screen and the casing. In the next step, a fluid slurry containing gravel is injected through the annulus and shunt tubes so that the fluid portion of the slurry is forced out of the annulus through the perforation tunnels into the reservoir and the gravel portion of the slurry is deposited in the annulus and forced out into the perforation tunnels into the formation. The last step is to terminate the injection of the fluid slurry containing gravel when the annulus is packed with gravel.
The method includes two additional steps, which relate more to how the apparatus is constructed than to how the gravel is packed. In accordance with one of these steps, the shunt tubes are positioned coaxially adjacent to the sand screen such that the shunt tubes extend substantially the length of the sand screen. In accordance with the other step, the cross-sectional area of the shunt tubes and the annulus are sized so that if gravel forms a bridge in a portion of the annulus thereby blocking the flow of fluid slurry through the annulus, the fluid slurry containing gravel will purportedly continue to flow through the shunt tubes and into the annulus around the gravel bridge.
While the method and apparatus disclosed in this prior art reference may help to divert the sand and fluid slurry mixture around any sand bridges, which may be formed, it does not reduce the incidence of sand bridge formation. Furthermore, the apparatus employed in this technique requires the use of additional shunt tubes secured to the outside of the sand screen, which block a portion of the sand screen. This has the disadvantage of blocking a portion of the passageway through which the hydrocarbons being captured is passed to the surface, and therefore reduces the rate, and thus the efficiency, at which hydrocarbons are produced from the reservoir. Another disadvantage of this system is that because the shunt tubes are mounted to the outside of the sand screen they are unprotected and thus susceptible to damage and dislodgment from the sand screen during the installation process. Yet another disadvantage of this configuration is that the external shunt tubes occupy space between the production or base pipe and the well casing or bore. Thus, either the screen base pipe needs to be smaller to accommodate the tubes, which results in less efficient production, or the wellbore has to be larger, which is undesirable.
At least two prior art devices have sought to solve the problem of damage to the shunt tubes. U.S. Pat. No. 5,515,915 issued to Jones et al. and also assigned to Mobil Oil Corporation, sought to solve this problem by placing the shunt tubes between the sand screen and the base pipe. U.S. Pat. No. 5,868,200 issued Bryant et al. and also assigned to Mobil Oil Corporation sought to solve this problem by placing a protective shroud over the shunt tubes. Both of these prior art devices, however, have the same drawbacks as the earlier patent owned by Mobil, namely they do not reduce the incidence of sand bridge formation and they are also not as efficient. The shunt tubes in these devices also block a portion of the passageway through which the hydrocarbons being captured are passed to the surface.